I've been seeing the same thing for a couple months, also someone else reported this in the 'Firefox bug' thread about the same time I first saw the problem. When you click on certain threads it seems to load the banner ad into a separate page, then you have to hit the back button to get back to the thread. I've only seen this on ET.
I'm using Firefox 3.0.3, I tried disabling various Add-ons including Adblock Plus, to no avail. Interestingly, I do not see this problem on my laptop which has the same Firefox version and similar configuration.